Antiperovskites (or inverse perovskites) is a type of crystal structure similar to the
perovskite structure
A perovskite is any material with a crystal structure following the formula ABX3, which was first discovered as the mineral called perovskite, which consists of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural mou ...
that is common in nature.
The key difference is that the positions of the
cation and
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
constituents are reversed in the
unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
structure. In contrast to perovskite, antiperovskite compounds consist of two types of anions coordinated with one type of cation. Antiperovskite compounds are an important class of materials because they exhibit interesting and useful physical properties not found in perovskite materials, including as electrolytes in solid-state batteries.
[Xia W, Zhao Y, Zhao F, et al. Antiperovskite Electrolytes for Solid-State Batteries. Chem Rev. 2022;122(3):3763-3819. ]
Structure
The crystal lattice of an antiperovskite structure is the same as that of the perovskite structure, but the anion and cation positions are switched. The typical perovskite structure is represented by the general formula ABX
3, where A and B are cations and X is an anion. When the anion is the (
divalent
In chemistry, the valence (US spelling) or valency (British spelling) of an chemical element, element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, ...
) oxide ion, A and B cations can have charges 1 and 5, respectively, 2 and 4, respectively, or 3 and 3, respectively.
In antiperovskite compounds, the general formula is reversed, so that the X sites are occupied by an
electropositive
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
ion, i.e., cation (such as an
alkali metal), while A and B sites are occupied by different types of anion. In the ideal cubic cell, the A anion is at the corners of the cube, the B anion at the
octahedral
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
center, and the X cation is at the faces of the cube. Thus the A anion has a coordination number of 12, while the B anion sits at the center of an octahedron with a
coordination number
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central io ...
of 6.
Similar to the perovskite structure, most antiperovskite compounds are known to deviate from the ideal cubic structure, forming
orthorhombic or
tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
phases depending on temperature and pressure.
Whether a compound will form an antiperovskite structure depends not only on its chemical formula, but also the relative sizes of the ionic radii of the constituent atoms. This constraint is expressed in terms of the
Goldschmidt tolerance factor
Goldschmidt's tolerance factor (from the German word ''Toleranzfaktor'') is an indicator for the stability and distortion of crystal structures. It was originally only used to describe the perovskite ABO3 structure, but now tolerance factors are a ...
, which is determined by the radii, r
a, r
b and r
x, of the A, B, and X ions.
Tolerance factor =
For the antiperovskite structure to be structurally stable, the tolerance factor must be between 0.71 and 1. If between 0.71 and 0.9, the crystal will be orthorhombic or tetragonal. If between 0.9 and 1, it will be cubic. By mixing the B anions with another element of the same valence but different size, the tolerance factor can be altered. Different combinations of elements result in different compounds with different regions of
thermodynamic stability
In chemistry, chemical stability is the thermodynamic stability of a chemical system.
Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibriu ...
for a given crystal symmetry.
Occurrence
Antiperovskites naturally occur in sulphohalite, galeite, schairerite,
kogarkoite, nacaphite,
arctite, polyphite, and hatrurite.
It is also demonstrated in
superconductive
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
compounds such as CuNNi
3 and ZnNNi
3.
Material properties
Synthesized Antiperovskites
Man-made antiperovskites exhibit interesting properties. The physical properties of antiperovskite compounds can be manipulated by altering the
stoichiometry, element substitution, and synthesis conditions.
Lithium Rich Antiperovskites (LiRAP)
Recently synthesized antiperovskites with chemical formula Li
3OBr and Li
3OCl have demonstrated high lithium-ion conductivity. Known as LiRAPs, these are being investigated for use as electrolytes in solid-state batteries and
fuel cells. In addition, other alkali-rich antiperovskites such as Na
3OCl are also being investigated for their
superionic conductivity.
Metallic Antiperovskite
Discovered in 1930, these crystals have the formula M
3AB where M represents a magnetic element, Mn, Ni, or Fe; A represents a transition or main group element, Ga, Cu, Sn, and Zn; and B represents N, C, or B. These materials exhibit
superconductivity,
giant magnetoresistance
Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter G ...
, and other unusual properties.
Antiperovskite manganese nitrides
Antiperovskite manganese nitrides have been shown to exhibit zero
thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
.
References
{{Reflist
Superionic Conductivity in Lithium-Rich Anti-PerovskitesLattice and Magnetic and Electronic Transport Properties in Antiperovskite CompoundsA material for all weathers (with zero thermal expansion) found in antiperovskite manganese nitrides
Mineralogy
Materials science